Acoustic echo cancellers (AECs) are used in communication systems, such as teleconferencing systems, to reduce the echo that results from the coupling between the loudspeaker(s) and microphone(s) at one end of a two way communication. For example, in many teleconferencing systems, the loudspeakers presenting the signal received from a far end are located close to the microphone for capturing sound at the local end. Accordingly, the sound produced by the loudspeakers is also picked up by the microphone and may therefore be returned to the far end resulting in a discernable echo at the far end. AECs are used to attenuate and preferable remove any contribution from the loudspeakers in the signal from the microphone.
AECs are often provided with the signals for the loudspeakers and then seek to estimate the resulting signal captured by the microphone. This predicted signal is then subtracted from the microphone signal. Typically, a single-channel (monophonic) system seeks to estimate the acoustic path response for the acoustic path from the loudspeaker to the microphone and use this estimation to generate the predicted signal. Thus, such an AEC simultaneously reduces the echo and identifies the acoustic path thereby ensuring that the echo remains cancelled no matter what happens at the remote end.
Multi-channel (e.g. stereo) teleconferencing systems have further been proposed where spatial information is captured at the remote end e.g. using two or more microphones. Such a system tends to provide a more realistic presence perception than a monophonic system since listeners can use the additional spatial information to facilitate e.g. audio scene analysis which is the process by which the brains reduces a sample of simultaneous audio streams into individual constituent sounds. Spatial information tends to result in increased intelligibility of speech signals in noisy surroundings and during double talk. It has also been found to reduce listener fatigue.
One of the main challenges in multi-channel echo cancellation is the strong coherence that exists between the input signals, making it hard or even impossible to correctly estimate the acoustic paths. For example, if the captured sound at the far end predominantly originates from a single speaker located equidistantly from the two microphones, the two resulting signals received at the near end may be virtually identical. Therefore, the individual contribution from each loudspeaker which is being captured by the microphone cannot easily be separated and therefore the individual acoustic path response of the acoustic path from each of the loudspeakers cannot be accurately estimated. In other words, it is not feasible to identify the individual contribution from the two loudspeakers.
To improve the performance of a multi-channel echo canceller, it has been proposed to reduce the coherence between the received signals by applying different time-varying allpass filters for the different channels. Thus, it has been proposed that each received signal from the far end is first filtered by a time-varying filter with the different filters varying differently. This introduces a decorralation of the signals radiated by each loudspeaker thereby allowing the individual acoustic paths to be estimated
Several approaches have been proposed to define and update the time-varying filters g1(q) . . . gM(q) such as e.g. using an allpass filter with changing characteristics. The filters are selected to provide an effective input decorrelation for an acceptable audio degradation as a result of introduced audio artifacts. Specific examples are described in the articles M. Ali, “Stereophonic Acoustic Echo Cancellation System Using Time-Varying All-Pass Filtering for Signal Decorrelation,” In Proc. of IEEE Int. Conf. on Acoustics, Speech, and Signal Processing (ICASSP), Vol. 6, pp. 3689-3692, May 1998; N. Tangsangiumvisai, J. A. Chambers and A. G. Constantinides, “Time-Varying Allpass Filters Using Spectral-Shaped Noise for Signal Decorrelation in Stereophonic Acoustic Echo Cancellation,” In Proc. of IEEE Asia Pacific Conf. on Circuits & Systems (APCCAS), Vol. 1, pp. 87-92, October 2002; and J. M. Valin, “Perceptually-Motivated Nonlinear Channel Decorrelation For Stereo Acoustic Echo Cancellation,” In Proc. of Joint Workshop on Hands-free Speech Communication and Microphone Arrays (HSCMA, May 2008.
However, although such decorrelation has been found to improve the channel estimation accuracy, the echo cancellation has been found to be suboptimal in many scenarios.
Hence, an improved acoustic echo cancellation approach would be advantageous and in particular an approach allowing increased flexibility, increased cancellation, facilitated implementation and/or improved performance would be advantageous.